The head of the Federal Communications Commission has expressed "serious concerns" about the planned merger between Sinclair Broadcast Group and Tribune TV stations, sending Sinclair's stock tumbling Monday.

"Based on a thorough review of the record, I have serious concerns about the Sinclair/Tribune transaction," FCC chairman Ajit Pai said in a statement Monday. "The evidence we've received suggests that certain station divestitures that have been proposed to the FCC would allow Sinclair to control those stations in practice, even if not in name, in violation of the law."

Pai called for a hearing in front of an administrative judge to decide the issue.

Sinclair is the nation's largest local broadcaster, reaching about 4 in 10 U.S. households through TV stations. It announced plans last year to merge with Chicago-based Tribune Media, which has 42 stations and stakes in CareerBuilder and the Food Network — a deal that would give Sinclair access to 72 percent of American households.

Critics have accused Sinclair of having a conservative bias, focusing on "must-run" segments that top company executives require local stations to air. In the early 2000s, Sinclair ran coverage favorable to the war in Iraq, according to Vox, and continued in later years with political ads that promoted conservative causes and candidates.

Top executives at Sinclair have donated to numerous Republican causes, including a super PAC supporting Mr. Trump's 2016 presidential bid.